The instant invention relates generally to nickel-metal hydride batteries and more specifically to high power nickel-metal hydride batteries. The batteries include negative electrodes which employ electrochemical hydrogen storage alloys with enhanced high discharge rate capacities, thereby increasing the specific power and high rate capabilities of the batteries. The negative electrode electrochemical hydrogen storage alloys include means to dramatically alter the discharge capacity thereof. As will be discussed hereinbelow in the Detailed Description of the Invention, these means include tailoring the local chemical and structural order of the materials by adding transition metals having d orbitals. This alteration results in discharge capacity curves for non-misch metal hydrogen storage alloys which are provide high capacity even at high discharge rates. Because the instant batteries can provide both high energy density and high power, they are uniquely suited for application in new uses and areas to which batteries were previously not apropos. A specific example of such means is Pd which increases the electrochemical hydrogen storage capacity at high discharge rates of high capacity non-misch metal alloys (misch metal alloys have inherently low hydrogen storage capacity.) These alloys have great advantages over prior art alloys which makes them particularly useful for electric vehicles, hybrid electric vehicles and have great utility for power tools and other high drain rate applications even ultracapacitors.
Presently batteries which can deliver high power and yet be small and light weight (i.e. have a high specific power) are in high demand. These types of batteries are useful in applications such as electric vehicle and hybrid electric vehicle propulsion for which lack of range has been a serious limitation. Also, in the electric vehicle industry, the high power capabilities allow for utilization of regenerative breaking to replenish the charge of the batteries. These high power batteries are also useful for other high drain rate applications such as power tools, as starter batteries for internal combustion engines and, because of the high conductivity of the metallic negative electrodes, as a replacement for power sources such as ultracapacitors.
Advanced automotive battery development for vehicle propulsion has, in the past, been directed primarily at the requirement of a true electric vehicle. Utilizing Ovshinsky""s principles of disorder, local order, and chemical modification, the hallmark Energy Conversion Devices, Inc. (xe2x80x9cECDxe2x80x9d), battery development teams at ECD and Ovonic Battery Company (xe2x80x9cOBCxe2x80x9d) have made great advances in nickel-metal hydride battery technology.
Initially Ovshinsky and his teams focused on metal hydride alloys that form the negative electrode. As a result of their efforts, they were able to greatly increase the reversible hydrogen storage characteristics required for efficient and economical battery applications, and produce batteries capable of high density energy storage, efficient reversibility, high electrical efficiency, efficient bulk hydrogen storage without structural changes or poisoning, long cycle life, and repeated deep discharge. The improved characteristics of these xe2x80x9cOvonicxe2x80x9d alloys, as they are now called, results from tailoring the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix, as well as the use of non-equilibrium processing. Disordered metal hydride alloys have a substantially increased density of catalytically active sites and storage sites compared to single or ordered multi-phase crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites which can themselves be increased no only by individual atoms but also by topology and chemistry. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at binding strengths within the range of reversibility suitable for use in secondary battery applications. A complete description of the role of disorder in electrochemical alloys is found in U.S. Pat. No. 4,623,597, the disclosure of which is incorporated herein by reference.
Some extremely efficient electrochemical hydrogen storage materials were formulated, based on the disordered materials described above. These materials reversibly form hydrides in order to store hydrogen. The materials are multiphase materials, which may contain, but are not limited to, one or more phases with C14 and C15 type crystal structures.
In contrast to the Ovonic alloys, the older alloys were xe2x80x9corderedxe2x80x9d materials that had homogeneous chemistry, a uniform microstructure, and generally poor electrochemical characteristics. In the early 1980""s, as the degree of modification increased (that is as the number and amount of elemental modifiers increased), their performance began to improve. Unbeknownst to the artisans of that era, who did all that was possible to keep the electrode materials uniform, the improvement in electrochemical performance was due as much to the compositional disorder contributed by the modifiers as it was to the electrical and chemical properties of the electrode alloys.
Simply stated, in all metal-hydride alloys, as the degree of modification increases, the role of the initially ordered base alloy is of minor importance compared to the properties and disorder attributable to the particular modifiers. In addition, analysis of the present multiple component alloys available on the market and produced by a variety of manufactures indicates that these alloys are modified following the guidelines established for Ovonic alloy systems. Thus, as stated above, all highly modified alloys are disordered materials characterized by multiple components and multiple phases, i.e. Ovonic alloys.
As a result of this development of the negative electrode active materials, the Ovonic Nickel Metal Hydride (Nixe2x80x94MH) battery has reached an advanced stage of development for EVs. Ovshinsky""s teams have been able to produce electric vehicle batteries which are capable of propelling an electric vehicle to over 350 miles on a single charge (Tour d"" Sol 1996). The Ovonic Nixe2x80x94MH battery has demonstrated excellent energy density (up to about 90 Wh/Kg), long cycle life (over 1000 cycles at 80% DOD), abuse tolerance, and rapid recharge capability (up to 60% in 15 minutes). Additionally, the Ovonic battery has demonstrated higher power density than any other battery technology under test and evaluation for use as an EV stored energy source.
While Ovshinsky and his teams have made great advances in batteries for true electric vehicles, the Partnership for a New Generation of Vehicles (PNGV), a U.S. government-auto industry partnership initiated in 1996, has suggested that hybrid-electric vehicles (HEV""s) could be the leading candidate to meet their goals of tripling auto fuel economy in the next decade. To realize this goal, lightweight, compact, high-power batteries would be required.
The use of a hybrid drive system offers critical advantages for both fuel economy and ultra-low emissions. Fuel engines achieve maximum efficiency when operating at constant rpm. Therefore, peak fuel efficiency can be achieved by employing a constant rpm fuel engine to provide energy to a high-power energy storage system that supplies peak power for acceleration and also recaptures kinetic energy through the use of regenerative braking.
Similarly, the ability to use a small engine operating at maximum efficiency and coupled with a pulse power energy storage system offers the best design for minimizing emissions associated with the use of a fuel engine. Therefore, a key enabling technology for HEV""s is an energy storage system capable of providing very high pulse power and accepting high regenerative braking currents at very high efficiency. The duty cycle of a pulse power application requires exceptional cycle life at low depths of discharge.
It is important to understand the different requirements for this energy storage system compared to those for a pure electric vehicle. Range is the critical factor for a practical EV, making capacity and discharge rate (power) critical evaluation parameters. Particularly in the HEV pulse power application, power density is the overwhelming consideration. Excellent cycle life under low depth discharge is also critical. Energy density is important to minimize battery weight and space, but due to the smaller battery size this characteristic is less critical than power density. Ability for rapid recharge is also essential to allow efficient regenerative braking, and charge/discharge efficiency is critical to maintain battery state of charge in the absence of external charging.
Given the fundamental differences in requirements between the EV and those for an HEV application, those batteries currently optimized for use in EV applications will not be suitable for HEV without an increase in power density. While the demonstrated performance of Ovonic EV batteries has been impressive, these cell and battery designs have been optimized for use in pure EVs and therefore do not meet the specific requirements for HEVs. Therefore, there is a need for high power batteries that have the peak power performance required by HEVs coupled with the already demonstrated performance characteristics and proven manufacturability of the Ovonic Nixe2x80x94MH batteries.
Previously Ovshinsky, et al. have provided nickel-metal hydride batteries and electrodes capable of increased power output and recharge rates that provide sufficient power for EV and HEV applications by providing nickel-metal hydride batteries having negative electrodes which were formed on porous metal substrates formed from copper, copper-plated nickel, or a copper-nickel alloy. These highly conductive substrates helped to improve the high power characteristics of the nickel-metal hydride batteries.
However, once conductivity of the substrate was improved, it became apparent that some of the negative alloy materials lacked the same high power capabilities, particularly with regard to high capacity at high rate of discharge. That is, it was noticed that the alloys which contained higher amounts of hydride forming elements, such as Cr, Ti and Zr, and lower amounts of Ni had reduced electrochemical hydrogen storage capacity at high discharge rates.
Relative thereto, U.S. Pat. No. 4,699,856 to Heuts et al. discloses misch metal-nickel hydrogen storage alloys into which Ni, Pd, Pt, Ir, and/or Rh was added to improve the low temperature rate performance of these misch metal type alloys. However, there is no teaching or suggestion to use a catalyst of any type in Tixe2x80x94Zrxe2x80x94Ni type alloys to improve the room temperature high discharge rate capacity thereof. This is particularly true because transition metal based alloys can inherently store a far greater amount of hydrogen than can their rare earth (misch metal) counterparts. Therefore, the misch metal alloys could not afford to lose any of their hydrogen storage capacity. However, there remains a need in the art to develop (transition metal) Tixe2x80x94Zrxe2x80x94Ni negative alloy materials which retain their high hydrogen storage capacity, at even higher discharge rates.
One object of the instant invention is an electrochemical hydrogen storage alloy including an effective amount of a catalytic transition metal to substantially increase the discharge capacity of the alloy at high discharge rates. This can be realized by an electrochemical hydrogen storage alloy comprising nickel, titanium, zirconium, vanadium and an effective amount of palladium to substantially increase the discharge capacity of the alloy at high discharge rates. An effective amount of palladium increases the discharge capacity of the alloy by at least 20% at a discharge of rate of C over the same alloy without palladium. The effective amount of palladium is generally from about 0.1 to 4 atomic percent of the alloy and more preferably 1 to 4 atomic percent. The alloys typically comprise, in atomic percent, 0.1 to 60% Ti, 0.1 to 40% Zr, 0 to 60% V, 0.1 to 57% Ni, 0 to 56% Cr, and 0 to 20% Mn and 0.1 to 4% Pd. Preferably the Ti, Zr, V, Ni, Cr, and Mn total at least about 80 atomic percent of said alloy.
Other objects of the present invention include negative electrodes formed with the alloys and nickel-metal hydride batteries formed with the negative electrodes.